The technique is known that collects a sample core of stratum. For example, in the technique, a drill bit is provided onto the circumference at the end of a cylindrical structure which is called a drill string. The drill string is rotated to invade the inside of stratum. After that, the drill string is extracted from the stratum, and a stratum sample of a column-like shape is collected from the inside of drill string. It becomes possible to know the physical characteristics such as the structure and space percentage of the stratum in detail by analyzing the stratum sample collected in this way. For example, this technique is expected to contribute to the oil and gas layer evaluation and the research of earthquake.
When the stratum is drilled with the drill string, there is a possibility that the phenomenon called kick or blowout occurs by stratum fluid flowing into a well. For example, when there are liquid layers such as an underground water vein and an oil and gas layer in a region where the tip of the drill string has reached, a possibility could be considered in which the stratum fluid flows backward from the drill bit depending on pore pressure in the stratum, passes through the inside of drill string, and blows out onto a ship or to the ground, so as to make the continuation of drilling difficult. When such kick and blowout seem to occur, it is desirable to take a well control measure in which a blowout preventing device is provided inside the drill string previously to prevent the kick and the blowout appropriately.
In relation to the above, inventions of a flapper-type float valve are disclosed in Patent Literature 1 and Patent Literature 2. In these inventions, a float valve using a flapper-type lid is provided inside the drill string in any case.